Proposals have been made for self-piloting check valve apparatus, in which dual valves are provided. One valve serves as a pilot valve, such as a spring-biased check valve, closing of which permits closing of a main valve, such as a ball valve, adapted to withstand elevated pressures. When fluid is flowing in the permitted direction the flow of fluid holds the check valve open and a spring biases the main valve to the open position. If there is no fluid flow the check valve closes, while the main valve remains open. If there should be a tendency for fluid to flow in the non-permitted direction, the pressure forces acting on the closed check valve are utilised to move the main valve to the closed position. Such valves have application in many situations.
The present invention will be described with reference to downhole uses in the oil and gas exploration and extraction industries, though the invention may of course be utilised in many other industries.